1. Field of the Invention
This invention relates to a power supply, and more particularly to a power supply for power factor correction and a driving method thereof wherein a ripple current of a power factor correction circuit (PFC) can be reduced to enhance a power factor and efficiency.
2. Description of the Related Art
Recently, there have been more activated applications of a power supply having stable, small size and light weight properties in a factory automation equipment, an office automation equipment, an information equipment, a communication equipment and a power supply, etc. Furthermore, a harmonic wave reduction and a power factor correction of an input current of the power supply have been issued as an important problem due to an expanded interest for a harmonic wave interference and an adoption of a harmonic wave limit standard.
Generally, a switching power supply can be divided into a rectifier for converting an alternating current (AC) input into a direct current (DC), and a DC to DC converter for stabilizing the DC input from a load variation and a change of input voltage. A capacitor input type rectifying circuit having largely been used as a DC power supply for various electronic equipments requires a large capacitance value of capacitor for the purpose of restraining an input voltage variation of the DC to DC converter to reduce a burden of a device.
However, as a capacitance value of a capacitor goes larger, a larger pulse-type current for storing a lot of energy for a short time flows. In this case, a peak value thereof becomes five to ten times larger than an effective value thereof. Since this large pulse-type current forces a shape of input current of the rectifier to be discontinuous, it makes an affect to peripheral devices due to a distortion of an input voltage and a harmonic component of an input current.
In order to overcome such problems, there has been suggested a scheme of adding a power factor correction circuit (PFC) to the DC to DC converter of the switching power supply. A circuitry for correcting a power factor in the existent power supply can be largely classified into a passive PFC and an active PFC.
Referring to FIG. 1, a power supply employing a conventional passive PFC circuit system includes a rectifier 10, consisting of four bridge diodes, for converting an AC input supplied from an AC voltage source Vin into a DC, an inductor L connected to one terminal of the rectifier 10, a capacitor C connected between the inductor L and other terminal of the rectifier 10, and a DC to DC converter 20 connected across the capacitor C to receive an energy stored in the capacitor C and convert it into a DC voltage.
Such a conventional power supply of passive PFC system limits a charge current of the capacitor C by an impedance value of the inductor L to thereby have an enlarged conduction angle and an improved power factor. The power supply of passive PFC system is mainly used for applications requiring a simple design, a low frequency band, a low electro-magnetic interference (EMI) and a high power.
Referring now to FIG. 2, a conventional power supply of active PFC system includes a rectifier 30, consisting of four bridge diodes, for converting an AC input supplied from an AC voltage source Vin into a DC, an inductor L connected to one terminal of the rectifier 30, a semiconductor switch Q1 connected between the inductor L and other terminal of the rectifier 30, a capacitor C between first and second terminals of the semiconductor switch Q1, a diode D1 connected to the first terminal of the semiconductor switch Q1 and the first terminal of the capacitor C, and a DC to DC converter 40 connected across the capacitor C to receive an energy stored in the capacitor C and convert it into a DC voltage.
The rectifier 30 rectifies an AC input supplied from the AC voltage source Vin using the bridge diodes, and stores it to the inductor L.
The semiconductor switch Q1 is controlled such that a current iL flowing in the inductor L follows an input voltage and a DC voltage Vc stored in the capacitor C is always constantly maintained at a larger value than a peak value of the maximum input voltage.
The DC to DC converter 40 receives an energy stored in the capacitor C and converts it into a required DC voltage depending upon a load connection (not shown).
As shown in FIG. 3, in such a power supply of active PFC system, an AC input is supplied from the AC voltage source Vin such that two diodes of the rectifier 30 are conducted during a half period of the AC input. At this time, if the semiconductor switch Q1 is turned on, an input voltage is applied to the inductor L to thereby linearly increase the current iL flowing in the inductor L and store an energy into the inductor L. At this time, a reverse voltage Vc from the capacitor C is loaded onto the diode D1 to thereby be turned off.
Then, if the semiconductor switch Q1 is turned off, then the diode D1 is conducted to apply a reverse voltage to the inductor L, thereby linearly decreasing the current iL flowing in the inductor L and storing the energy having been stored in the inductor L into the capacitor C. The energy Vc stored in the capacitor C is fed to the DC to DC converter 40 to be converted into a required DC voltage.
In such a conventional power supply of active PFC system, the semiconductor switch Q1 allows the DC voltage Vc stored in the capacitor C to be always constantly maintained at a larger value than a peak value of the maximum input voltage. In the case of a power supply supplying a power to a plasma display panel, the DC voltage Vc is maintained at approximately 380V to 400V, and is set to have an always lager value than 265V because the maximum value of the input voltage is 265V.
However, in this case, the current iL is linearly increased when the input voltage is applied to the inductor L, whereas the current iL is decreased when a difference between the output voltage and the input voltage of the capacitor C is applied thereto. Accordingly, as shown in FIG. 4, when an input voltage supplied via an input line is low, a difference between the DC voltage stored in the capacitor C and the input voltage is enlarged and hence a pulsation of the current iL flowing in the inductor L is enlarged. As the pulsation of the current iL is enlarged, a conduction loss of the semiconductor switch Q1 is increased and a power loss caused by resistance components of each device, thereby deteriorating a whole efficiency of the power supply. Furthermore, it needs a relatively large EMI filter due to the enlarged pulsation of the current iL. Moreover, as the pulsation of the current iL is enlarged, a current control characteristic is deteriorated to reduce a power factor.